than EAs (Sartor et al., 2013; Vaughn et al., 2008). Notably, these variations in prevalence and sequence may relate to differing societal attitudes towards cannabis and cigarette use, the relative availability and exposure opportunity of the two drugs as well as to putative differences in biological response to anticipation and receipt of drug-related rewards. For cigarette use, we are only aware of 3 studies, including two by us in the sample under study here, that show that additive genetic factors explain similar proportions of variance (40–50%) in AAs and EAs (Sartor et al., 2009, 2015; Whitfield et al., 2007). However, in a recent study by our group (Sartor et al., 2015), the remainder of the variance in cigarette use was solely attributable to individual-specific environmental factors (44%) in AA twins while in EA twins, substantial influence of both individual-specific (10%) and shared environmental factors (34%) was noted for cigarette use. Likewise, we have previously reported that timing to cannabis use is heritable in AA female twins (0.52) and that the role of shared environment is limited (Sartor et al., 2009). However, no study to date has examined the bivariate relationship between lifetime use of cannabis and cigarettes in AA and
